1. Field of the Invention
The present invention relates to a boot, which covers constant-velocity universal joints, an indispensable component part for the joint of drive shaft for front-wheel-drive vehicle, to inhibit water and dust from coming in the joint element of constant-velocity universal joint.
2. Description of the Related Art
The joint element of constant-velocity universal joint is covered with a bellows-shaped boot in which grease is sealed. The boot inhibits water and dust from coming in the joint element to enable a constant-velocity universal joint to rotate smoothly with large angles. A boot for constant-velocity universal joint comprises a major-diameter fastener, a minor-diameter fastener, and a substantially truncated cone-shaped bellows. The major-diameter fastener is fitted around the major-diameter retainer of cup, a mating member. The minor-diameter fastener has a minor diameter, which is smaller than that of the major-diameter fastener, and is fitted around the minor-diameter retainer of shaft, another mating member. The bellows connects the major-diameter fastener with the minor-diameter fastener integrally. When the boot is put in service, the bellows deforms in compliance with the changing angle, which the major-diameter fastener and minor-diameter faster make. As a result, even when a constant-velocity universal joint rotates with a large angle, the boot can cover the joint element of the constant-velocity universal joint securely.
In order to make conventional boots for constant-velocity universal joint be likely to follow the changing angle, which the central axis of cup and the central axis of shaft make, and in order to make the stress, which occurs in their bellows, small, conventional boots therefor are formed so that the crests of the bellows have a large diameter comparatively. However, it has been required recently to reduce the weight of vehicle. As the size of vehicle body is made more compact, it has been required as well to make the configuration of boots for constant-velocity universal joint more compact. When a boot for constant-velocity universal joint is made more compact, it is possible not only to reduce the weight of boot per se, but also to reduce the amount of grease, which is held inside boot. Therefore, a compacted boot contributes to the weight reduction of automobile greatly.
On the assumption that the overall length of boot for constant-velocity universal joint is kept constant, it is needed and is indispensable to diminish the outside diameter of bellows in order to make a boot for constant-velocity universal joint more compact. When diminishing the outside diameter of bellows, the membrane length of the crests is shortened accordingly. Hence, in order to keep the displacement magnitude and arising stress equivalent to those exhibited by conventional boot for constant-velocity universal joint, it is necessary to make the depth of the roots of bellows greater.
However, simply making the depth of the roots of bellows greater might result in such a drawback that, when the central axis of cup and the central axis of shaft make a large angle, the roots of bellows, which are disposed adjacent to the major-diameter fastener, are bit between the cup and shaft. When the roots are thus bit, a larger force acts onto the roots. If the drawback occurs repetitively, the damaged roots impair the sealing ability of boot. Accordingly, it is inevitable to replace boots frequently. Consequently, a problem arises that the thus compacted boot has a shortened longevity.
In connection therewith, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 4-300,463 discloses a boot whose bellows comprises six or more crests and 5 or more roots. Moreover, the bottoms of 3 or more roots, which are disposed on the major-diameter fastener side, make a line, which forms a major-diameter root pattern; and the bottoms of 2 or more roots, which are disposed on the minor-diameter fastener side, make a line, which forms a minor-diameter root pattern. In this conventional boot, the bellows bends so as to follow the two patterns, the major-diameter root pattern and minor-diameter root pattern. Accordingly, the conventional boot produces good bendability especially at the central portion, the boundary between the two patterns. Moreover, the patent publication sets forth that the conventional boot does not undergo any unreasonable deformation and/or distortion in the circumstances of high-speed rotation so that it is good in terms of the durability.
Moreover, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2002-295, 509 proposes a boot for constant-velocity universal joint, boot which comprises a straight section disposed between the crest, disposed closest to the minor-diameter fastener, and the minor-diameter fastener. This conventional boot makes it possible to reduce the number of crests, which are disposed on the side of minor-diameter fastener and which exhibit less displacement magnitude, by one. Therefore, it is possible to reduce the weight of boot without adversely affecting the durability of boot.
In addition, from the viewpoint of the readiness of mold making and the man-hour requirements of molding, it is desirable to mold the bellows of boot for constant-velocity universal joint by blow molding. However, in the case of making the overall length of boot constant, it becomes difficult to blow molding such a boot because the pitches between the crests and the roots narrow down when the number of the crests and roots increases. On the other hand, when the number of the crests and roots is less, the resulting boot has exhibited lowered degree of deformation freedom in service.
Hence, the “preliminary compression” has been carried out conventionally. That is, the overall length of boot is formed longer than the assembly length by increasing the number of the crests and roots; and the resulting boot is assembled with a constant-velocity universal joint while being compressed. Thus, it is possible to secure the membrane lengths in bellows sufficiently. Accordingly, the resultant boot can secure the durability with ease. However, when the difference between the dimension upon molding and the dimension upon assembling enlarges, the load required for preliminary compression has enlarged excessively to deteriorate the assembly workability.
In particular, an in-board side boot for constant-velocity universal joint is exposed to a higher temperature than an out-board side boot for constant-velocity universal joint is. Moreover, an in-board side boot expands and contracts in the axial direction more frequently than an out-board side boot does, because the minor-diameter retainer moves in the axial direction relatively with respect to the major-diameter retainer. It is for these reasons that an in-board side boot is required to satisfy furthermore sufficient durability.